Membrane reactors are known in the art. For example, WO2010/086635 discloses a reactor wherein a synthesis gas is subjected to a water-gas shift reaction, and carbon dioxide is separated from the shifted gas mixture by a CO2-selective membrane, thereby generating a hydrogen-enriched gas mixture.
WO2004/022480 describes a process and apparatus for the production of high purity hydrogen by steam reforming. The apparatus is an integrated reactor for steam reforming of a hydrocarbon to produce H2 and CO2, with minimal CO in the H2 stream. The reactor contains multiple flameless distributed combustion (FDC) chambers and multiple hydrogen-selective, hydrogen-permeable, membrane tubes. The feed and reaction gases flow through the reactor radially or axially. This document also describes different configurations of baffles which may be employed in the multi-tubular, FDC heated, axial flow, membrane steam reforming reactors to increase contact of the reactant gases with the catalyst in the catalyst beds. The baffle configuration comprises a washer shaped baffle and a disk shaped baffle arranged in and alternating pattern. This baffle arrangement causes the feed and reactant gases to flow through the hole in the washer shaped baffle and be deflected by disk shaped baffle thereby enhancing the contact of the reactant gases with the catalyst which is packed in the area between the baffles. Another baffle arrangement comprises truncated disks which are placed in and alternating pattern (truncated left and truncated right) in the reactor thereby causing the feed and reactant gases to “zigzag” as they flow through the catalyst which is packed in the area between the baffles. The baffles will have openings to allow the FDC tubes and membrane tubes to pass through them. Screens positioned in vertical alignment may also be used to support the baffles and in some cases hold the catalyst away from the shell wall or from the centre of the shell for better gas flow distribution.
In prior art membrane reactor designs, such as described in WO2004/022480, the catalyst and membrane are fully integrated. That is to say, that the catalyst may be a packed bed surrounding the membranes or that the catalyst is coated on the membrane.
In such prior art membrane reactor designs, the catalyst is distributed more or less evenly along the membrane, and the flow length of the feed through the catalyst bed is of the same order as the membrane length. In the case of an endothermic reaction, heat required for the reaction is provided, for example, through burners or heat exchanging elements arranged as repetitive elements with the membranes. On the other hand in the case of exothermic reaction, heat formed may be removed though heat exchanging elements. Consequently the catalyst volume, the heat exchanging area and membrane area cannot be selected independently in the design. Given the inherently low flux of the reaction or side product to be removed, for example hydrogen, through a given membrane area, compared to the rate in which it can be produced in a certain catalyst volume, and the heat flux required to enable the reaction to produce the reaction product, in prior art membrane reactor designs there is a mismatch between the catalyst volume, heat transfer area and membrane area.
An option to decouple the catalyst volume and the membrane area is to carry out separation and reaction process sequentially (alternating reaction and separation).
WO2009/150678 uses separate vessels containing membranes for separation and vessels containing the catalyst for reaction sequentially (membrane-catalyst-membrane-catalyst-membrane-catalyst, etc), whereby reactor and separation units and catalyst can be sized independently. However, from a practical point of view this is suitable only for a process in which a limited number of steps is necessary. Calculations (see examples) for steam reforming have shown that a limited number of sequential steps, for example 5 or less, the conversion of feed which can be achieved, and thus the economic attractiveness, is limited. Further, it will require a large amount of piping and control systems to connect the vessels in series.
Because membrane reactor processes operate at high pressures and temperatures, the containment, i.e. the pressure vessel which contains the membrane/catalyst assemblies, may become very costly. The most important reason seems that the flow rate through the membranes (flux) is low. The current invention allows a close packing of the membranes, a more compact art of designing the reactor.
In addition, mass transfer is an issue in packed membrane reactors. It is known (F. Gallucci et al. (2010), Theoretical comparison of packed bed and fluidized bed membrane reactors for methane reforming, International J. Hydrogen Energy 35/13, pp. 7142-7150) that mass transfer adversely influences the performance of membrane reactors, for example in the tube-in-tube concept (such as described in WO2004/022480). Because the concepts described such as the tube-in-tube concept lack the degree of freedom to choose the velocities independently, the mass transfer cannot be enhanced by increasing the gas velocities.
Hence, disadvantages of the prior art are the complicated solutions, the non-compact solutions, and the sub-optimal use of catalyst (more catalyst seems to be used than necessary).